1. Field of the Invention
This invention generally relates to a system for removing sulfur dioxide from flue gases by scrubbing with an alkaline liquid and, more particularly, this invention relates to a sulfur dioxide scrubbing system wherein the absorbent liquid is a sodium-based solution which is continuously regenerated outside the scrubbing apparatus by treatment with a calcium compound, such as lime or slaked lime.
2. State of the Art
Sulfur dioxide is a gas which is created by the oxidation of sulfur-bearing materials such as coals, oil, and natural gas. While it has long been deemed desirable to limit the concentration of sulfur dioxide in combustion gases which are released to the atmoshpere, no completely commercially-satisfactory, dependable sulfur dioxide removal system has yet been devised. Sulfur dioxide emission is a particularly acute problem in the electric power generating industry where large quantities of coal are burned.
It is well known that sodium-based scrubbing solutions, such as sodium hydroxide in water, have a great affinity for sulfur dioxide. Since such solutions are relatively expensive, attempts have been made to regenerate the sodium-based scrubbing liquors by reacting, or causticizing, the spent scrubbing solutions with an alkaline earth compound, such as lime or limestone. Such a regenerating process is taught, for example, in U.S. Pat. No. 1,271,899 to Howard wherein a dilute solution of sodium sulfite leaving a scrubbing apparatus is reacted with lime to produce calcium sulfite as a precipitate and to regenerate the sodium hydroxide scrubbing liquor.
In practice, such regenerative sulfur dioxide treatment processes have not operated satisfactorily. One problem has been the presence of calcium ions in the regenerated scrubbing solution; those ions tend to produce insoluble deposits, which are called "scale" and which consist essentially of calcium sulfate (gypsum) plus some amounts of calcium sulfite, which foul the scrubbing apparatus and plug the associated piping. One reason the calcium ions are present in the regenerated scrubbing solution is because the recausticizing reaction produces, inter alia, calcium sulfate, an ionic compound which is relatively highly soluble.
Some art workers in such regenerative processes have attempted to reduce the calcium ion concentration by trying to avoid the formation of calcium sulfate. To do that, those workers have attempted to minimize the degree of oxidation of the scrubbing liquid in the scrubbing apparatus or in the lines leading from the apparatus, to thereby minimize the formation of sodium sulfate in favor of sodium sulfite formation. Other art workers have accepted that calcium sulfate is formed during the regenerative process and have then attempted to remove the calcium sulfate prior to re-using the recausticized scrubbing liquor. These latter workers have used a combination of steps to accomplish the calcium sulfate removal; first, they have maintained an excess of sodium sulfate in the regenerating reaction zone to encourage the precipitative removal of calcium sulfate and, subsequently, they have treated the causticized scrubbing solution with large amounts of sodium carbonate, thereby to eliminate most of the remaining calcium cations in the form of a precipitate, calcium carbonate. In practice, however, the techniques of the latter art workers have loaded the entire scrubbing systems with sodium ions which not only is uneconomical but requires an excessive purge to maintain chemical control.